3-34 Basic DMM Operation Model 2700 Multimeter/Switch System User’s Manual
Reference junctions
A reference junction is the cold junction in a thermocouple circuit which is held at a
stable, known temperature. It is at the cold junction where dissimilar wire connections
must be made. As long as the temperature of the cold junction is known, the Model 2700
can factor in the reference temperature to calculate the actual temperature reading at the
thermocouple.
The standard reference temperature is the ice point (0°C). The ice point can be precisely
controlled, and the National Bureau of Standards uses it as the fundamental reference for
its voltage-to-temperature conversion tables. However, other known temperatures can be
used.
There are two ways for the Model 2700 to acquire the cold junction temperature. It can
measure the cold junction using a thermistor or 4-wire RTD, or the known temperature
value can be entered by the user.
There are three reference junction types supported by the Model 2700: simulated reference
junction, internal reference junction, and external reference junction. These reference
junctions are explained in the following paragraphs.
NOTE When using multiple channel operation (ROUT:MULT command) to connect a
switching module input channel to the DMM, the SIMulated reference junction
will be used if the INTernal or EXTernal reference junction is selected.
Simulated reference junction
An example of a simulated reference junction is an ice bath (Figure 3-14A and B). The
copper wire to thermocouple wire connections are immersed (but electrically isolated) in
the ice bath, and the user enters the 0°C simulated reference temperature into the Model
2700. The simulated reference temperature for the Model
2700 can be set from 0 to 65°C.
The Model 2700 measures the input voltage and factors in the simulated reference
temperature to calculate the temperature reading at the thermocouple.
NOTE The most accurate temperature measurements are achieved by using a simulated
reference junction using an ice point reference.
Internal reference junction
“Internal” implies that a temperature transducer(s) is used to measure the cold junction.
For the Model 7700 switching module, the cold junction is the screw terminals, with
voltage temperature sensors strategically placed to measure the temperature of the cold
junction.
The Model 2700 measures the temperature of the cold junction (screw terminals),
measures the input voltage, and then calculates the temperature reading at the
thermocouple.
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